In a randomized clinical trial of succimer, an oral chelating agent that lowers blood lead concentration, children with blood leads of 20 to 44 micrograms per deciliter were given succimer or placebo when they were about 2 years old and followed with cognitive and behavioral testing. We reported previously that, despite lower blood lead levels during treatment, children given succimer had test scores that were no different from those of children given placebo at ages five or seven years. This year we used the trial data set to determine whether the effects of blood lead on IQ were determined by peak blood lead at age 2 or by lower blood lead at later ages. We found that, contrary to most current thinking, that concurrent blood lead bore a stronger relation to IQ than did peak blood lead, and that the relation between peak blood lead and IQ attenuated as the children got older, while the cross sectional association grew stronger. We were asked by the American Academy of Pediatrics to be the primary author on their new lead poisoning policy statement.